Multiple inheritance in javascript

Question

Here's some question about oop in js (questions in the code below).

<html>
    <script>
    function A(){
      a = 'a - private FROM A()';
      this.a = 'a - public FROM A()';
      this.get_a = function(){
        return a;
      }
    }

    function B(){
      this.b = 'b - private FROM B()';
      this.a = 'a - public FROM B() ';
    }

    C.prototype = new A();
    C.prototype = new B();
    C.prototype.constructor = C;
    function C() {
      A.call(this);
      B.call(this);
    }

    var c = new C();

    //I've read paper about oop in Javacscript but they never talk 
    //(the ones have read of course) about multiple inheritance, any 
    //links to such a paper?

    alert(c.a);
    alert(c.b);
    alert(c.get_a());

    //but

    //Why the hell is variable a from A() now in the Global object?
    //Look like C.prototype = new A(); is causing it.

    alert(a);

    </script>
</html>

I guess I should clarify my question : What are the drawback of multi-inheritance in js? And why variable a is in global scope? — plehoux
– plehoux, Commented Sep 22, 2010 at 15:10

slebetman · Accepted Answer · 2010-09-22 15:16:27Z

7

C.prototype = new A();
C.prototype = new B();

Multiple inheritence isn't supported in javascript. All you've done is make C inherit from B instead of A.

answered Sep 22, 2010 at 15:16

slebetman

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MooGoo · Accepted Answer · 2010-09-22 15:14:21Z

6

You can't. When you do this

C.prototype = new A();
C.prototype = new B();

You are merely changing the object that prototype points to. So C used to inherit from A, but now it inherits from B.

You can fake multiple inheritance

C.prototype = new A();

for (var i in B.prototype)
  if (B.prototype.hasOwnProperty(i))
    C.prototype[i] = B.prototype[i];

Now you'd have the properties/methods from both A and B, but you don't really have inheritance, since any changes to the prototype object of B will not propagate to C.

answered Sep 22, 2010 at 15:14

MooGoo

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Golmote Kinoko · Accepted Answer · 2010-09-22 15:14:14Z

3

You need to declare the variable a with the var statement in order to make it local to the function.

function A(){
  var a = 'a - private FROM A()';
  this.a = 'a - public FROM A()';
  this.get_a = function(){
    return a;
  };
}

answered Sep 22, 2010 at 15:14

Golmote Kinoko

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